Fibers made from elastic materials find use in a variety of applications ranging from woven fabrics to spunbond elastic mats to disposable, personal hygiene items. It would be of particular interest to use styrenic block copolymers for such applications. However, the typical phase-separated nature of block copolymer melts leads to high melt elasticity and high melt viscosity. In order to process styrenic block copolymers through small orifices, such as found in fiber spinnerets, expensive and specialized melt pump equipment would be required. Further, the high melt elasticities lead to fracture of the fiber as it exits the die, preventing the formation of continuous elastomeric fibers. As a result, styrenic block copolymers have been found to be exceedingly difficult to process into continuous elastic fibers at high processing rates.
A further problem with styrenic block copolymers is their inherent stickiness in the melt. Because of this character, melt spun fibers of styrenic block copolymers tend to stick together, or self-adhere, during processing. This effect is not desired and can be, in fact, tremendously problematic when separate, continuous fibers are the goal. In addition to the result of an unacceptable fiber product, the self-adhesion of the fibers leads to equipment fouling and expensive shut-downs. Efforts to apply styrenic block copolymers in elastic fiber production have to date been met with significant challenges.
Himes taught the use of triblock/diblock copolymer blends as one approach to make elastomeric fibers in U.S. Pat. No. 4,892,903. These types of compositions have been found to have high viscosities and melt elasticities which have limited them to formation of discontinous and continuous fibers such as used in melt-blown, non-woven applications.
Bicomponent fibers comprising acid functionalized styrenic block copolymers have been taught by Greak in European Patent Application 0 461 726. Conventional, selectively hydrogenated styrenic block copolymers which were acid functional were used to form side-by-side bicomponent fibers with polyamides. While the acid functionalization provided increased adhesion between the two components, it is well known in the art that acid functionalization leads to even higher melt viscosities and melt elasticities than in unfunctionalized block copolymers. Further, the side-by-side morphologies taught by Greak would not prevent the inherently sticky fibers from self-adhering during processing.
Bonded non-woven webs made using bicomponent fibers comprising, among other polymers, conventional styrenic block copolymers and having a variety of morphologies has been taught by Austin in U.S. Pat. No. 6,225,243. In particular, the sheath-core morphologies, with the core being comprised of styrenic block copolymers, provided fibers of suitably low stickiness to form non-woven webs.
However, high viscosity and melt elasticity of conventional styrenic block copolymers continues to prevent high speed spinning of continuous elastomeric fibers. The present invention addresses these longstanding needs by providing a high melt flow block copolymer which is able to be formed into continuous elastomeric bicomponent fibers.